1. Field of the Invention
This invention relates in general to the construction of respirating devices and in particular to a new and useful device for regulating the operation of the respirator in accordance with a function of the partial pressure of the carbon dioxide in the arterial blood or the tissue through the alveolar expiration air.
2. Description of the Prior Art
The present invention particularly deals with a device for controlling the respiration in respirators as a function of the partial pressure of CO.sub.2 in the arterial blood or the tissue through the alveolar expiration air. Up to the present time the mechanism of the respiration control for regulating the respirating air in accordance with its partial pressure of CO.sub.2 is not known completely. It is certain however that in respiration the partial pressures of CO.sub.2 and oxygen are of substantial importance. In this connection a control of the ventilation on the basis of the arterial CO.sub.2 content has proved more appropriate than on the basis of the oxygen concentration.
With a supply of air which is too small the partial pressure of CO.sub.2 that is the P.sub.CO.sbsb.2 value in the patient's blood increases. This value is balanced with the CO.sub.2 content in the alveolar respiration air. A too large amount of inspiration air results in a low partial pressure of CO.sub.2. Known respirators determine the partial pressure or content in the expiration air and control the respiration gas regime of the patient in accordance with a predetermined standard. A known respirator comprises a pressure control device with which, by means of a signal from the CO.sub.2 meter, the pressure of the inspiration air supplied to the patient is either increased or decreased. With an inspiration air volume which is too small and a correspondingly high CO.sub.2 level in the blood, a pressure increase causes a deeper and stronger inspiration. Due to the appropriately induced expiration the partial pressure of carbon dioxide in the blood is reduced. This process continues up to the optimum relation between the volume of the inspiration air and the CO.sub.2 level in the blood. Inversely, the pressure in the inspiration air is reduced if the volume is larger than the optimum. With a less deep respiration, the CO.sub.2 level will increase. Thus in this respirator the CO.sub.2 content is controlled by a pressure variation, with a larger or smaller volume of respiration air resulting therefrom. The apparatus needed for such a purpose is complicated and must be firmly integrated. Such a respirator can be used only for patients in good health having a normal perfusion-to-ventilation ratio. If this ratio is disturbed, for example, in the presence of an embolism, low expiration CO.sub.2 values are obtained which in the respirator described would lead to a reduction of the respiratory volume. Such a reduced respiratory volume is incapable of washing out the CO.sub.2 produced and this leads to a respiratory acidosis within a short period of time.
Another known respirator controls the oxygen supply through the CO.sub.2 content of the expiration air in accordance with the patient's needs. Here again the output of the carbon dioxide meter is compared with a reference value. A differential signal thus produced actuates a tapping valve to change the oxygen content in the inspiration air and the depth of the respiration is remedied. The oxygen supply is controlled within the limits predetermined for the CO.sub.2 concentration. The control device comprises a feedback system largely employing electronic component parts. From the physiological point of view, the control of the volume of respiratory air by an oxygen supply is disputed. The electronic equipment of the control system is extensive and its maintenance is not simple.